Wanted: "kill my children" button in the Ketrew web UI

[iskandr]

Currently, if we kill a node then it's child nodes keep on running. To actually kill the entire tree of dependencies we have to first find them all via the filter query language.

It would be significantly more convenient to instead have a button attached to each target "Kill My Children".

This, like a "Find My Failed Children" button (https://github.com/hammerlab/ketrew/issues/345), implies the creation of a new (child-of node-id) predicate.